It is known to monitor a condition of a vehicle component, such as tire pressure. This may be accomplished wirelessly. In such tire pressure monitoring systems, each tire has an associated tire pressure sensor and a radio frequency (“RF”) transmitter mounted to its wheel so as to be on the inside of the tire. The tire pressure sensed by each tire pressure sensor is transmitted by its associated RF transmitter to a receiver located onboard the vehicle. The tire pressure information delivered to the on-board receiver via the RF signals from each of the transmitters is subsequently conveyed to a vehicle operator or occupant, typically in the form of a display in the vehicle occupant compartment.
The RF transmitters used in such tire pressure monitoring systems are typically battery powered and are energized for limited operating times so as to extend the life of the transmitter batteries. The transmitters usually transmit tire pressure information at time intervals that are shorter when the vehicle is moving and longer when the vehicle is not moving.
The vehicle-mounted receiver in such tire pressure monitoring systems can either assume an inactive state when the vehicle ignition is OFF or can alternate between active and inactive states to reduce current draw on the vehicle battery: As a result, the tire pressure information from the tire-mounted transmitters often must include a wake-up signal to ensure receipt of the transmitted pressure signal by the vehicle-mounted on-board receiver. The requirement to transmit an additional wake-up signal will shorten the life of the transmitter battery. A wake-up signal, however, is transmitted only when the vehicle has been stationary for a predetermined period of time. Motion of the vehicle is generally detected by a tire motion sensor in each tire.